public class Solution {
    
    public List<String> getFlipResult(String s1) {  
        List<String> res = new ArrayList<String>();  
        if(s1.length()<2) return res;  
        dfs(s1, res, 0);  
        return res;  
    }  
    public void dfs(String s1, List<String> res, int len) {  
        for(int i=len;i<s1.length()-1;i++) {  
            if(s1.charAt(i) == '+' && s1.charAt(i) == s1.charAt(i+1)) {  
                String t1 = s1.substring(0, i) + "--" + s1.substring(i+2);  
                res.add(t1);  
            }else{  
                dfs(s1, res, i+1);  
            }  
        }  
    }  
    
}